Optical devices such as optical modulators or displays employ electro-optical elements that rely on modification of light absorption induced by an electrical signal. For example, U.S. Pat. Nos. 3,854,795, 5,016,990, and 7,016,094 provide electro-optical devices such as switches, filters, and tunable lasers, which are characterized by the modification of an absorption coefficient induced by an electrical-signal. Electrical-signal induced modification of absorption of electromagnetic waves in devices employing a semiconductor material may be obtained by injected charges, tuning of the band gap (Franz-Keldysh-effect), or the quantum-confined Stark effect in quantum wells. Electrical-signal induced modification of absorption of electromagnetic waves in devices employing ferromagnetic perovskite materials may be obtained by the magnetoresistive effect.
The main drawback of the above-mentioned electro-optical elements employing a semiconductor material is “volatility” of the change in the absorption characteristics of the semiconductor material. In other words, the change in the absorption characteristics of the semiconductor material is effective only as long as electrical power needs to be applied to the optical element. Such electro-optical elements are “volatile,” i.e., they do not maintain the characteristics of a programmed state once power to the electro-optical element is turned off. A further disadvantage of the prior art optical elements employing a semiconductor material is that such electro-optical elements typically employ silicon or germanium, and are thus only suitable for the infrared wavelength range.
The main drawback of the above-mentioned electro-optical elements employing a ferromagnetic perovskite material is that these elements are only suitable for the far-infrared wavelength region. Specifically, such electro-optical elements are not suitable for the telecommunication wavelength range, which include 1,340 nm and 1,550 nm. Further, such electro-optical elements are not suitable for the visible wavelength region between 400 nm and 800 nm because the band gap of such materials is about 0.5 eV.
A further disadvantage of prior art electro-optical elements is that crystalline materials suffer from polarization dependency (birefringence), which requires additional structures to enable polarization-independent device operation. Moreover, crystalline materials, especially quantum wells, require high temperature processing, and therefore cannot be integrated back-end-of-line (BEOL) metal interconnect structures.
A non-volatile electro-optical element utilizing changes in refractive index of a metal oxide material is disclosed in co-assigned European Patent Application No. 08100566.2. This electro-optical element provides modulation of phase of the light by employing oxygen vacancies. Because the change in the refractive index is relatively small, this device tends to require a long optical path.
In view of the above, there exists a need for a compact electro-optical element that provides non-volatile modulation of absorption characteristics of an optical medium, methods of manufacturing the same, and methods of operating the same.
Further, there exists a need for such a compact electro-optical element that may be integrated into BEOL metal interconnect structures and does not have birefringence, methods of manufacturing the same, and methods of operating the same.